1. Field of the Invention
This invention is related to an automatic transmission control system for automobiles and, in particular, to a control system for controlling the supply of coupling pressure on frictional coupling elements of an automatic transmission through a hydraulic pressure circuit.
2. Description of Related Art
In automatic transmissions for automobiles, a transmission gear mechanism is coupled to a torque converter, and a drive torque transmission path of the transmission gear mechanism is changeable through the selective coupling of a plurality of frictional coupling elements so as to automatically shift the automatic transmission to various gears according to vehicle driving conditions. In such an automatic transmission, a hydraulic controlling circuit controls coupling pressure supplied to the frictional coupling elements. Japanese Unexamined Patent Publication No. 1-150055 discloses a hydraulic control circuit of this type, which has a linear solenoid valve to control coupling pressure supplied to each of the frictional coupling elements. This hydraulic control circuit includes a regulating valve disposed in a pressure line to the frictional coupling elements. The control pressure supplied to the regulating valve is developed by means of the linear solenoid valve. In this instance, a control signal is output to the linear solenoid valve from a controller so as to develop the control pressure supplied to the regulating valve, thereby appropriately controlling the coupling pressure applied to the frictional coupling elements. For example, through the adjustment of torque transmission capacity of the frictional coupling element relative to an input torque to the frictional coupling element, the frictional coupling element reliably transmits torque without loss in driving force of an oil pump attributable to an unnecessary increase in torque transmission capacity.
In addition, with this type of hydraulic control circuit, when supplying coupling pressure to the frictional coupling element during gear shifting, in order to prevent the transmission from causing "shift shock" due to an abrupt coupling of the frictional coupling element or elements, the coupling pressure is gradually increased. Hence, an accumulator is necessarily provided in a hydraulic pressure line to the frictional coupling elements.
The accumulator functions to delay the rising or implementation of the coupling pressure supplied to the frictional coupling elements through the hydraulic pressure lines by causing the movement of a piston of the accumulator against a back pressure within the hydraulic pressure line. However, properly controlling the rising or implementation of coupling pressure causes the necessity of controlling the back pressure continuously acting on the piston. For this control, a linear solenoid valve or a duty solenoid valve is necessarily installed in the hydraulic pressure line. Accordingly, as described in the above mentioned publication, where a solenoid valve is provided to regulate the coupling pressure itself along with the regulating valve, then another solenoid valve is required to control the back pressure of the accumulator. This results in an increase in the number of solenoid valves that must be provided in the hydraulic control circuit. In addition, the control pressure generating solenoid valve is not required to have a high flow rate of output but essentially regulates the pressure supplied to the regulating valve with a high accuracy. On the other hand, since the back pressure control solenoid valve must provide the back pressure of the accumulator prior to introducing the coupling pressure to the accumulator, it is required to have a high flow rate of output. These conflicting requirements preclude use of a single solenoid valve for providing the functions of both control pressure generating solenoid valve and back pressure control solenoid valve. Consequently, the control valve units included as part of the hydraulic control circuit must be large in size, with a significant increase in weight and cost.